Illumination shares no less than 20% of civilian electric energy consumption, which accounts for 1.9 GT of CO2 emissions. (Taguchi, T., IEEJ Trans. 2008, 3, 21.) The ever-increasing energy demands and the concerns of global warming are pressing for the development of high-efficiency light sources to reduce energy consumption. Solid-state lighting (SSL) in the form of light-emitting diodes (LEDs) can convert electricity to light much more effectively than conventional lighting sources. It has been predicted that a nation-wide move toward SSL for general illumination in the U.S. could save 32.5 quads of primary energy between 2012 and 2027. Therefore, high efficiency LEDs are being explored intensely, especially white LEDs (WLEDs), which have been considered to be a potential light source to replace conventional lighting systems such as fluorescent lamps and incandescent bulbs.
There are currently two major WLED systems: multi-chip WLEDs and one-chip WLEDs. (Taguchi, T., Ieej Trans. 2008, 3, 21.) In the multi-chip WLEDs, white light is created by combining three LED chips with colors of red (R), green (G), and blue (B), respectively. Since each LED requires a power source, and each source has its own specific lighting characteristic, balancing their luminous intensity to obtain an even color mixture is a challenging task and often results in inadequate illumination. Uchida, Y.; Taguchi, T., Opt. Eng. 2005, 44, 124003. In addition, RGB multi-chip LEDs are most expensive. Different from the multi-chip WLEDs, the one-chip WLEDs consist of a LED (blue, near-ultraviolet, or ultraviolet) and phosphors, namely phosphor-converted WLEDs (pc-WLEDs). The first commercialized pc-WLEDs are constructed by combining the blue InGaN chip with the yellow YAG:Ce phosphor (Fasol, G.; Nakamura, S. The Blue Laser Diode:GaN Based Blue Light Emitters and Lasers; Springer: Berlin, 1997.), in which the blue light from the LED excites the YAG:Ce phosphor to emit yellow light, which is subsequently mixed with the blue light to generate white light. However, these WLEDs have the problem of achieving a high color rendering index of over 85 due to their red spectral deficiency. (Mueller, A. H., et al., Nano Lett. 2005, 5, 1039.) Compared with the RGB WLEDs and blue-YAG WLEDs, the near-ultraviolet or ultraviolet LED pumped WLEDs fabricated by UV-LED chips coated with white light-emitting single-phased phosphors or RGB tri-color phosphors may overcome the aforementioned shortcomings owing to the invisible emission of the LED chip, and thus, have been considered an important and promising future direction of SSL technology. (Zhang, Q. Y., et al., Mater. Sci. Eng. R 2010, 71, 1.) Although NUV-LED+RGB phosphors represent one of the best white light assemblies with both high luminous efficiency and high CRI, RGB phosphors obtained by mixing three phosphors with colors of red, green, and blue, respectively, suffer from complex blending of different phosphors, lack of efficient red phosphors, and self-absorption. (Yang, W. J., et al., Chem. Mater. 2005, 17, 3883.) Pc-WLEDs with single-phased white-emitting phosphors can eliminate the need of complex color mixing or conversion techniques, enabling easy fabrication with perfect color reproducibility, stability and high efficiency. Clearly, the development of this kind of pc-WLEDs depends on the breakthrough of the study on single-phased white-emitting phosphors.
Among several kinds of single-phased white-light emitting phosphors developed in recent years for use in WLEDs including organic molecules and inorganic nanomaterils, semiconductor nanocrystals (NCs) are an intensively explored group because of their size-dependent optical and electronic properties, cost-effective solution-based processability, and high quantum yield. (Dai, Q. Q., et al., Small 2010, 6, 1577.) A large number of white-light emitting NC systems have been synthesized, including ZnS:Pb, ZnSe, “magic-sized” CdSe, Mn-doped CdS, Mn-doped ZnS, trap-rich CdS, onionlike CdSe/ZnS/CdSe/ZnS, and alloyed ZnxCd1-xSe. In particular, the “magic-sized” CdSe NCs (average diameter less than 2 nm) have become a topic of intensive interest. Unlike traditional NCs (diameters larger than 2 nm) that exhibit near-monochromatic band-edge photoluminescence, the magic-sized CdSe NCs emit a broad white light that covers the entire visible spectrum as a consequence of very high surface-to-volume ratio which leads to a significantly large number of midgap surface sites. Most recently, WLEDs based on the electroluminescence of the magic-sized CdSe NCs that have excellent color characteristics and high color rendering indexes are reported. (Schreuder, M. A., et al., Nano Lett. 2010, 10, 573. However, these WLEDs suffer from very low luminous efficiency (˜1.0 lm/W). (Dai, Q. Q., et al., Small 2010, 6, 1577.) The weak correlations among the quantum dots make it difficult to achieve high conductivity and mobility required for a LED. Semiconductor bulk materials that have good transport properties and can convert electricity directly to white light are most desirable.
As an alternative lighting source, solid state lighting (SSL) technologies (primarily light emitting diodes and organic light emitting diodes) have received considerable attention in recent years due to their enormous potential for use in lighting and displays. The major advantages of SSL are lower energy consumption, higher efficiency, and longer lifetime. White light emitting diodes (WLEDs) are of particular interest because of the great need in general illumination applications. Common approaches to produce WLEDs include blending of three primary colored LEDs, namely red, green, and blue (RGB) diodes, or combination of a blue (or UV) LED with a yellow phosphor (or multiphosphors). Either process requires complex doping/mixing and delicate control of multiple materials and colors, which proves both challenging and costly.
At the present time, commercially available WLEDs are predominantly phosphor based (e.g. a yellow emitting phosphor, yttrium aluminum garnet or (YAG):Ce3+, coupled with a blue emitting InGaN/GaN diode). While less expensive than the RGB diodes, the (YAG):Ce3+ type phosphors and WLEDs have issues such as inaptness for solution process, poor color rendering index (CRI) and high correlated color temperature (CCT), and more impotantly the issues of rare-earth element (REE) supply shortage, which limit their widespread commercialization in general lighting market. Semiconductor quantum dots (QDs) or nanocrystals (NCs) with broad and strong absorption and tunable emission are promising alternative phosphors because they are solution processable. However, their emission bands are often too narrow. White light obtained by combining blue-, green-, and red-emitting QDs of various sizes, on the other hand, often suffers from low efficiency caused by self-absorption, scattering and related energy transfer issues. In addition, it is of great difficulty and complexity to control the size of QDs and maintain an appropriate amount of each component to balance the color. WLEDs based on QDs/NCs typically have relatively low quantum efficiency (QE), e.g., 2% for Mn-doped CdS NCs and 2-3% for magic sized CdSe NCs.
These problems may be reduced or eliminated by developing (a) more complex QDs/NCs composites or (b) single-phased white light emitters in bulk form. For (a), significantly improved quantum efficiency of 30% for onion-like CdSe/ZnS/CdSe/ZnS, 17% for trap-rich CdS, 17% for Cu:Mn—ZnSe, and 12% for alloyed [ZnxCd1-xSe], have been achieved. However, in most cases, multiple steps are involved in the synthesis, and precise control of NC core and/or shell size remains highly challenging. In addition, surface modification is often required, which adds further complexity to the synthesis process. For (b), on the other hand, there are only very few known examples of single-phased white light emitters in bulk form, for example, [[AgL]n.nH2O] (L=4-cyanobenzoate), which has a quantum yield (QY) of 10.9%, in addition to several single-phased organic white light emitting materials.